1. Field of the Invention
This invention concerns an improved process for carbonylating methanol to acetic acid.
2. Description of the Prior Art
Among currently-employed processes for synthesizing acetic acid one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. These compounds are employed as promoters, not stabilizers. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
Paulik et al teach that the liquid reaction medium can be any solvent compatible with the catalyst system and that it may comprise, for example, the pure alcohol which is being reacted, or mixtures thereof with the desired carboxylic acid end product and/or esters of these two compounds. However, the patentees teach further that the preferred solvent and liquid reaction medium for the process is the desired carboxylic acid itself, i.e., acetic acid when methanol is being carbonylated to produce acetic acid. Paulik et al also disclose that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate.
Considering specifically the carbonylation of methanol to acetic acid in a solvent comprising predominantly acetic acid and using the promoted catalyst taught by Paulik et al, it is taught in European patent application 0,055,618 that about 14-15 wt. % water is present in the reaction medium of a typical acetic acid plant using this technology. Recovering acetic acid in anhydrous or nearly anhydrous form from such a reaction solvent and separating the acetic acid from this appreciable quantity of water, involves substantial expenditure of energy in distillation and/or additional processing steps such as solvent extraction, as well as enlarging some of the process equipment as compared with that used in handling drier materials. Also Hjortkjaer and Jensen [Ind. Eng. Chem., Prod. Res. Dev. 16, 281-285 (1977)] have shown that increasing the water from 0 to 14 wt. % water increases the reaction rate of methanol carbonylation. Above about 14 wt. % water the reaction rate is unchanged.
Another disadvantage of the carbonylation process as just previously described is that the rhodium catalyst tends to precipitate out of the reaction medium, especially during the course of distillation operations to separate the product from the catalyst solution when the carbon monoxide content of the catalyst system is reduced (EP0,055,618). It has now been found that this tendency increases as the water content of the reaction medium is decreased. Thus, although it might appear obvious to try to operate the process of Paulik et al at minimal water concentration in order to reduce the cost of handling reaction product containing a substantial amount of water while still retaining enough water for adequate reaction rate, the requirement for appreciable water in order to maintain catalyst activity and stability works against this end.
Other reaction systems are known in the art in which an alcohol such as methanol or an ether such as dimethyl ether can be carbonylated to an acid or ester derivative using special solvents such as aryl esters of the acid under substantially anhydrous reaction conditions. The product acid itself can be a component of the solvent system. Such a process is disclosed in U.S. Pat. No. 4,212,989 issued July 15, 1980 to Isshiki et al, with the catalytic metal being a member of the group consisting of rhodium, palladium, iridium, platinum, ruthenium, osmium, cobalt, iron, and nickel. A somewhat related patent is U.S. Pat. No. 4,336,399 to the same patentees, wherein a nickel-based catalyst system is employed. Considering U.S. Pat. No. 4,212,989 in particular, the relevance to the present invention is that the catalyst comprises both the catalytic metal, as exemplified by rhodium, along with what the patentees characterize as a promoter, such as the organic iodides employed by Paulik et al as well as what the patentees characterize as an organic accelerating agent. The accelerating agents include a wide range of organic compounds of trivalent nitrogen, phosphorus, arsenic, and antimony. Sufficient accelerator is used to form a stoichiometric coordination compound with the catalytic metal. Where the solvent consists solely of acetic acid, or acetic acid mixed with the feedstock methanol, only the catalyst promoter is employed (without the accelerating agent), and complete yield data are not set forth. It is stated, however, that in this instance "large quantities" of water and hydrogen iodide were found in the product, which was contrary to the intent of the patentees.
European published patent application No. 0,055,618 to Monsanto Company discloses carbonylation of an alcohol using a catalyst comprising rhodium and an iodide or bromine component wherein precipitation of the catalyst during carbon monoxide-deficient conditions is alleviated by adding any of several named stabilizers. A substantial quantity of water, of the order of 14-15 wt. %, was employed in the reaction medium. The stabilizers tested included simple iodide salts, but the more effective stabilizers appeared to be any of several types of specially-selected organic compounds. When an iodide salt is used as the stabilizer, the amount used is relatively small and the indication is that the primary criterion in selecting the concentration of iodide salt to be employed is the ratio of iodide to rhodium. That is, the patentees teach that it is generally preferred to have an excess of iodine over the amount of iodine which is present as a ligand with the rhodium component of the catalyst. Generally speaking the teaching of the patentees appears to be that iodide which is added as, for example, an iodide salt functions simply as a precursor component of the catalyst system. Where the patentees add hydrogen iodide, they regard it as a precursor of the promoter methyl iodide. There is no clear teaching that simple iodide ions as such are of any significance nor that it is desirable to have them present in substantial excess to increase the rate of the reaction. As a matter of fact Eby and Singleton [Applied Industrial Catalysis, Vol. 1, 275-296 (1983)] from Monsanto state that iodide salts of alkali metals are inactive as cocatalyst in the rhodium-catalyzed carbonylation of methanol.
Carbonylation of esters, such as methyl acetate, or ethers, such as dimethyl ether, to form a carboxylic acid anhydride such as acetic anhydride is disclosed in U.S Pat. No. 4,115,444 to Rizkalla and in European patent application No. 0,008,396 by Erpenbach et al and assigned to Hoechst. In both cases the catalyst system comprises rhodium, an iodide, and a trivalent nitrogen or phosphorus compound. Acetic acid can be a component of the reaction solvent system, but it is not the reaction product. Minor amounts of water are indicated to be acceptable to the extent that water is found in the commercially-available forms of the reactants. However, essentially dry conditions are to be maintained in these reaction systems.
U.S. Pat. No. 4,374,070 to Larkins et al teaches the preparation of acetic anhydride in a reaction medium which is, of course, anhydrous by carbonylating methyl acetate in the presence of rhodium, lithium, and an iodide compound. The lithium can be added as lithium iodide compound. Aside from the fact that the reaction is a different one from that with which the present invention is concerned, there is no teaching that it is important per se that the lithium be present in any particular form such as the iodide. There is no teaching that iodide ions as such are significant. This patent further discloses feeding 2 to 7 volume % hydrogen to the reactor to suppress tar formation and significantly increase the reaction rate in terms of methyl acetate conversion as well as acetic anhydride production.
On the other hand, in an article in Chem Tech, Vol. 1, Oct. 1971, pages 600-605, James F. Roth et al describe the results of many experiments conducted on the carbonylation of methanol to acetic acid apparently utilizing the Monsanto commercial process. This process is essentially that described in previously mentioned U.S. Pat. No. 3,769,329 (Paulik et al). Among the results found was that the addition of hydrogen did not affect reaction rate. As hydrogen exerted no ill effect on the product composition, hydrogen was characterized as an inert diluent. Such finding is consistent with the disclosure in U.S. Pat. No. 3,769,329 wherein it is disclosed that carbon monoxide streams containing inert impurities such as hydrogen, carbon dioxide, methane, nitrogen, noble gases, water, and light paraffinic hydrocarbons may be employed from an available plant gas stream with no adverse effect, although in such cases total reactor pressure will have to be increased to maintain a desired carbon monoxide partial pressure. Concentrations of carbon monoxide in the feed gas mixture can be from 1 vol. percent to 100 vol. percent. Moreover, Paulik et al disclose that the rhodium-halogen catalyzed carbonylation process is readily adaptable to purifying hydrogen streams of carbon monoxide impurities since the carbon monoxide in such a gas mixture readily undergoes reaction with an alcohol, for example butyl alcohol to yield pentanoic acid.
Another patent describing the use of hydrogen in carbonylation reactions is U.K. patent application GB 2,155,929, published Oct. 2, 1985. In this particular patent, a process for producing acetic anhydride by the carbonylation of methyl acetate comprises separating the reaction mixture formed in the carbonylation reaction step into a volatile component and a rhodium-containing catalyst solution, heat treating the separated catalyst solution in the presence of a hydrogen-containing gas, and recirculating the hydrogenated catalyst solution to the carbonylation reaction step. It has been found that the activity of the rhodium catalyst was recovered. The patent specification states that the invention is also useful for the carbonylation of methanol.
In Japanese application 82-163034, Early Disclosure No. 59-53440, published Mar. 28, 1985 is disclosed a process for simultaneously preparing acetic anhydride and acetic acid comprising carbonylating a mixture of methyl acetate and methanol in a liquid phase containing the presence of rhodium, a 3-substituted phosphine, chromium or zirconium, and an iodine compound. The carhon monoxide feed may contain 1 to 10 mol % H.sub.2 to affect the selectivity of reaction and the life of catalytic activity. The molar ratio of methyl acetate to methanol in the mixture is disclosed as ranging from 1/3 to 3/1. A methyl acetate-methanol mixture containing an excessive amount of methanol used as feed is to be avoided since the amount of produced acetic anhydride is small and thus, would deviate from the object of the invention as well as be troublesome to separate and obtain acetic anhydride from the reaction mixture. A methyl acetate-methanol mixture containing water, if in a small amount, is disclosed as being useful as the feed.
A similar process for the anhydrous co-production of acetic acid and acetic anhydride is disclosed in European patent application 170,965, published Feb. 12, 1986 and assigned to Hoechst A.G. A catalyst system comprising rhodium and a phsophonium compound is used along with an organic iodide. Hydrogen can be added to the reaction medium in amounts of about 10 volume percent.
European patent applications 144,935 and 144,936, published June 19, 1985 and assigned to Union Carbide Corporation, disclose processes for the production of carboxylic acids from alcohols such as the production of acetic acid from methanol. The two European patent applications disclose processes for the production of acetic acid by the catalytic reaction of methanol and carbon monoxide in contact with methyl acetate and a homogeneous catalyst system containing rhodium metal and lithium iodide, and rhodium metal and a mixture of lithium iodide and methyl iodide, respectively. Both of the published European patent applications state that the invention disclosed therein does not require the use of acidic halogen promoters as it employes the alkali metal halide lithium iodide, nor does the invention require the presence of water or use of large quantities of methyl iodide to give selectivity to acetic acid as taught in U.S. Pat. No. 3,769,329 to Monsanto. Other than this broad statement, the published European patent applications to Union Carbide are silent on the effect of water on reaction rate.
An improvement in the prior-art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in copending, commonly assigned application U.S. Ser. No. 699,525, filed Feb. 8, 1985, now abandoned and European patent application 161,874; published Nov. 21, 1985. As disclosed therein acetic acid (HOAc) is produced from methanol (MeOH) in a reaction medium comprising methyl acetate (MeOAc), methyl halide, especially methyl iodide, (MeI), and rhodium present in a catalytically-effective concentration. The invention therein resides primarily in the discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4 wt. % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt. % or 15 wt. % water) by maintaining in the reaction medium, along with a catalytically-effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, and a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The application teaches that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt. %, so low that it can broadly be defined simply as "a finite concentration" of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product-recovery steps of the process wherein distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Ser. No. 699,525 is herein incorporated by reference.
In summary, with exception to the improved carbonylation process as described in commonly assigned U.S. Ser. No. 699,525, current technology in the field of carbonylating an alcohol such as methanol to form a carboxylic acid such as acetic acid lacks a simple method for maintaining a highly stable catalyst system and for attaining industrially attractive conversion rates at conditions of low water content in the liquid reaction medium whereby the expense and capital investment costs of recovering the acid product with a very low water content can be minimized. Even with the improved carbonylation process as described in commonly assigned U.S. Ser. No. 699,525, there is still a need to increase reaction rates.
It is, accordingly, an object of the present invention to provide a reaction system with which an alcohol, as exemplified by methanol, can be carbonylated to a carboxylic acid derivative such as acetic acid at an increased carbonylation rate above that which is disclosed in commonly assigned U.S. Ser. No. 699,525 using a liquid reaction medium having a lower water content than heretofore considered feasible. It is another object to provide a catalyst system which, regardless of the water content of the reaction medium, will be of improved stability, i.e., more resistant to precipitation of solid catalyst therefrom. It is also a related object to provide a catalyst system characterized by a substantial reduction in the undesired formation of by-product propionic acid and carbon dioxide as compared with high water systems used in the prior art. Other objects will be apparent from the following description.